Field of the Invention
The invention relates to a nuclear magnetic flow meter for measuring the flow rate of a multiphase medium which is flowing through a measuring tube, with a nuclear magnetic measurement device, the nuclear magnetic measurement device being located around the measuring tube. Moreover, the invention also relates to a method for operation of nuclear magnetic flow meters.
Description of Related Art
The atomic nuclei of the elements which have a nuclear spin also have a magnetic moment which is caused by the nuclear spin. The nuclear spin can be construed as an angular momentum described by a vector, and accordingly the magnetic moment can also be described by a vector which is aligned parallel to the vector of the angular momentum. The vector of the magnetic moment of an atomic nucleus in the presence of a macroscopic magnetic field is aligned parallel to the vector of the macroscopic magnetic field at the location of the atomic nucleus. Here the vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of the precession is called the Larmor frequency ωL and is proportional to the amount of the magnetic field strength B. The Larmor frequency is computed according to the relationship ωL=γB. Here, γ is the gyromagnetic ratio which is maximum for hydrogen nuclei.
Measurement methods which influence the precession of atomic nuclei of a medium in the presence of a macroscopic magnetic field by excitation by means of a controlled magnetic field and which evaluate the action of the influence are called nuclear magnetic resonance measurement methods. Usually, the electrical signals which have been induced in a sensor coil by the precessing atomic nuclei after excitations are used as the output variable for the evaluation. The prerequisite for the measurement of a multiphase medium is that the individual phases of the medium can be excited to distinguishable nuclear magnetic resonances. The magnitude of the electrical signals which have been induced in the sensor coil by the precessing atomic nuclei of a phase of the medium is dependent on the number of precessing atomic nuclei per volumetric element in this phase, accordingly therefore dependent on the density of the phase, but also dependent on the duration of influence of the precessing atomic nuclei in the influencing controlled magnetic field. Consequently, the magnitude of the electrical signals for the liquid phases of the medium is larger than for the gaseous phases. It follows from this that, in nuclear magnetic flow meters, the measurement accuracy for the measurement of the liquid phase of the medium can be relatively accurate, while the smaller magnitude of the electrical signals for the gaseous phases adversely affects the measurement accuracy of the nuclear magnetic flow meters for the measurement of the gaseous phase, especially when the gaseous phase has a relatively low density and/or when the gaseous phase is flowing with a relatively high velocity through the measuring tube.
One example of measurement devices which use nuclear magnetic resonance is the initially addressed nuclear magnetic flow meters whose nuclear magnetic measurement devices can measure the flow rate, that is, the flow velocity of the individual phases of the medium and the relative proportions of the individual phases in the multiphase medium. Nuclear magnetic flow meters can be used, for example, for measuring the flow rate of a multiphase medium which has been conveyed from oil sources. This medium consists essentially of the liquid phases crude oil and salt water and the gaseous phase natural gas, all phases containing the hydrogen nuclei which are necessary for nuclear magnetic resonances and being excitable to different nuclear magnetic resonances.
In the measurement of the medium which has been conveyed from oil sources, it is also possible to work with test separators. The conveyed medium is introduced into test separators over a time interval and the test separators separate the individual phases of the medium from one another and determine the proportions of the individual phases in the medium. However, test separators, in contrast to nuclear magnetic flow meters, are not able to reliably separate proportions of crude oil smaller than 5%. Since the proportion of crude oil of all sources continuously decreases and the proportion of crude oil of a host of sources is already less than 5%, at present it is not possible to economically exploit these sources using test separators. In order to furthermore also be able to exploit sources with a very small proportion of crude oil, correspondingly accurate flow meters for the medium, crude oil, which consists of several phases are necessary. In particular, nuclear magnetic flow meters are possible for this purpose.